It is useful to know the location of people or objects for several reasons. The location of people or objects allows another party to find lost people or objects, such as a child or expensive equipments. Location information can also be used as a piece of data in conjunction with other information. For example, knowledge about the location of a portable laptop computer combined with knowledge about the location of all the printers in a building can allow a system to automatically route a print job from the laptop computer to the nearest printer, thus saving time and aggravation. Additionally, the knowledge of who is in a particular room in a building can also allow a system to adjust the temperature or lighting of that room to the individual's preferences or route that person's telephone calls to the phone in that room. These applications are examples illustrating the utility of a system that allows the location of people or objects to be known.
Conventional location systems are generally based on one of two methods. In the first method, the amount of time is measured for a signal to travel from point A to point B, and then the distance between the two points A and B is calculated. In the second method, the conventional location systems calculate the distance between a transmitter and a receiver based on a received signal strength indication (RSSI). The RSSI is a function of distance and a path-loss factor:RSSI=1/d−fwhere d is distance and f is the factor.
While the second method is conceptually simpler than the first method, the signal wave travels over many paths between the transmitter and the receiver, especially in indoor space. Sometimes these multiple path signals arrive at the receiver in-phase (constructively) and sometimes they arrive at the receiver out-of-phase (destructively). This means that the RSSI can be 3 dB higher than the actual value or as much as 30 dB lower than the true value. This multiple path fading makes it extremely difficult to determine the RSSI value accurately in the location systems that are based on the second method.